The present invention relates to wipe generators, and more particularly to a digital wipe generator having anti-aliasing to maintain sub-pixel resolution and having improved circle generation capability.
In the video broadcast arts a wipe is a transition, normally accomplished in a video production switcher, between two video sources that follows a selected geometric pattern. A wipe generator creates the selected geometric pattern by deriving a waveform that then controls video mixers in video switching circuits. Traditional wipe generators, such as that shown in FIG. 1, have a waveform generator section, a solid generator section, a matrix generator section and a clip and gain section. The waveform generator section has horizontal (H) and vertical (V) counters, a rotation multiplier matrix, and absolute value circuits that produce waveforms X and Y. The X and Y waveforms are typically ramp and/or triangle waveforms that can be inverted and/or rotated. The solid generator section contains arithmetic circuits that combine the X and/or Y waveforms in various ways to produce a third waveform WS called a "solid". The clip and gain section performs a traditional operation upon the solid to produce the wipe waveform where the clip point pertains to the spatial position of the wipe transition and the gain pertains to the softness of the wipe transition. The matrix section produces "matrix" wipes that are a look-up table derived wipe waveform, the look-up table being addressed by the X and Y waveforms.
Current wipe generators are generally of analog architecture. However a digital wipe generator is required for use with a digital switcher. The straight forward approach would be to take the output of the analog wipe generator and digitize the analog signals with appropriate analog to digital (A/D) converters. This approach results in noise caused distortions, i.e., circle patterns lose their shape when made small. Another approach is to take the analog architecture and convert directly to a digital design. Current attempts at a digital architecture have resulted in severe aliasing problems, i.e., the edges of lines that are not exactly horizontal or vertical demonstrate a staircase effect commonly called "jaggies".
What is desired is an anti-aliased digital wipe generator that maintains sub-pixel resolution and constant shape wipe patterns at all sizes.